In pipeline and process instrumentation, electronic control loops are often used to sense physical process variables and correspondingly actuate a control device such as a valve. Most analog or digital process control systems use currents variable between 4 and 20 milliamperes for the transmission of control signals or the control of field devices. Typically, a current to pressure transducer receives a 4 to 20 ma DC input signal from a controller and transmits a proportional 3 to 15 psig pneumatic output pressure to a final control element such as a control valve. Changes in ambient temperature, supply pressure changes, mounting positions, and other influences require that the current/pressure transducer zero set point and the input/output range or span of the transducer be adjusted in compensation for such influences.
Prior zero set point adjustment techniques utilized mechanical means such as the addition of a zeroing spring which acts on a pneumatic flapper-nozzle balance system to cause a desired compensating change in the output pressure. Other mechanical zero adjustments utilize changes in the nozzle position or by changing the torsion of flexure members to adjust the output pressure. Mechanical span adjustments also have been utilized involving a change in the position of a pressure feedback device or by means of a magnetic shunt. Another technique in span adjustment has been to use a potentiometer in parallel with the current carrying coil of the current to pressure transducer.
Such prior zero and span adjustment techniques require mechanical linkages to the current/pressure transducer; make remote adjustments difficult if not impossible; significantly deteriorate the system vibration performance; and require temperature compensation by thermistors which can only be an approximation for one specific resistance value of the prior art span adjusting potentiometer.
Accordingly, it is desired to provide zero and span adjustment means for a current/pressure transducer which do not require any mechanical linkages to the transducer, and if desired, can be installed remotely to the current to pressure transducer to thereby provide significant flexibility in the control loop system design.